The present invention relates generally to magnetic disk drives and, more particularly, to a method of tuning feed-forward control in a disk drive.
Magnetic disk drives generally read and write data on the surface of a rotating magnetic disk with a transducer that is located at the far end of a moveable actuator. A servo control system uses servo control information recorded amongst the data, or on a separate disk, to controllably drive a voice coil motor (VCM) that moves the transducer from track to track (xe2x80x9cseekingxe2x80x9d) and to hold the transducer at a desired position (xe2x80x9ctrack followingxe2x80x9d). A detailed discussion of servo control systems is unnecessary because such systems are well known as set forth, for example, in patent application Ser. No. 09/138,841 that was filed on Aug. 8, 1998, now U.S. Pat. No. 6,204,988 entitled xe2x80x9cDISK DRIVE CAPABLE OF AUTONOMOUSLY EVALUATING AND ADAPTING THE FREQUENCY RESPONSE OF ITS SERVO CONTROL SYSTEM,xe2x80x9d and is commonly owned by the assignee of this application.
Most disk drives have previously been used for storing conventional data files of the type that are associated with personal computers. In such applications, data integrity is paramount relative to other considerations such as seek times and the reduction of acoustic noise. It presently appears, however, that disk drives are likely to become popular for recording and replaying audiovisual dataxe2x80x94e.g. a drive based recording device that replaces video cassette recorders (VCRs). A drive-based recording device of this nature will benefit from using a disk drive with faster seek times because it will spend less time moving its actuator where it needs to be and have more time to record or recover information in any given unit of time. The drive-based recording device, therefore, may be able to record and/or playback more audiovisual data streams that otherwise possible. At the same time, the drive-based recording device is likely to be located adjacent to a television or be in some other location where acoustic noise issues arise. Accordingly, it is equally important for the disk drive to implement its seeks in as quiet a manner as possible.
The drive industry has progressed through several stages of development as related to seeks. Of particular relevance here, is the prior use of a so-called xe2x80x9cbang-bangxe2x80x9d seek profile wherein the transducer is rapidly accelerated at the start of a seek and then rapidly decelerated at the end of a seek using saturated levels of command effort (current or voltage). A bang-bang seek profile moves the transducer to a target position in as rapid a manner as possible. On the other hand, because the bang-bang acceleration profile is a square wave, it is relatively loud and it contains many high frequency components that may detrimentally excite a mechanical resonance that causes the transducer to take longer to settle into the target position. It has previously been determined that a quieter, faster settling seek is possible by xe2x80x9cshapingxe2x80x9d the transducer""s acceleration profile so that it does not appear like a square wave, but rather approximates a single frequency sine wave. The result is a shaped seek profile that is xe2x80x9cclosexe2x80x9d to a bang-bang square wave but is quieter and does not contain the high frequency components that may excite the drive""s resonant frequencies.
Modem disk drives generally use a sampled servo control system that only periodically receives position information (e.g. once per servo sector) and shortly thereafter outputs a command effort based on a deviation between the indicated position and the target position. The servo control system in such a drive implements a shaped seek profile as a feed-forward profile using a feed-forward control path as a feed-forward profile using as discussed,for example, in co-pending patent application Ser. No. 09/538,931 pending that was filed on Mar. 31, 2000, entitled xe2x80x9cDISK DRIVE WITH FEED-FORWARD CONTROL PATH THAT RECEIVES A REFERENCE POSITION SIGNAL TO APPLY A FEED-FORWARD COMMAND EFFORT AT A RATE GREATER THAN A SERVO SAMPLING RATE,xe2x80x9d (the xe2x80x9cMulti-rate Feed-Forward Applicationxe2x80x9d), which application is commonly owned by the assignee of this application and hereby incorporated by reference in its entirety.
Modeling errors in the feed-forward control path, however, may cause the servo to inaccurately follow the intended feed-forward profile. In the Multi-rate Feed-Forward Application, the feed-forward control path models the VCM and overall plant as a rigid body that responds equally to all input frequencies in terms of magnitude and phase, i.e. it implements a simple double derivative of the PES reference signal to form a sinusoidal seek profile without regard to the frequency-dependent variance of the VCM. Obviously, in such case, the effectiveness of the feed-forward control path will diminish to the extent that the actual VCM response G(z) differs from the modeled VCM response G0(z).
The most prevalent modeling errors are gain errors due to variation in the values of the motor torque constant (KT) and the motor winding resistance (RW) due to changes in temperature, calibration errors, track pitch errors, and other factors. These modeling errors may cause the actuator to either overshoot or undershoot the target position at the end of the shaped seek, and thereby increase the required settling time for seeks of all lengths.
One could simply rely on the feedback loop that contains the feedback control path to repeatedly, reactively reign in any deviation caused by the feed-forward control path""s modeling error in a sample-by-sample fashion (as in the above-referenced Multi-rate Feed-forward application), but system performance would be improved if the effect of the modeling error could be eliminated or reduced.
One approach to addressing the deviation between actual and modeled frequency response is the inclusion of an adaptive control path that characterizes the difference and dynamically accounts for it during each seek. One such system monitors the error signal e during the first several samples of a seek in order to characterize any modeling error between the modeling VCM response G0(z) and the actual VCM response G(z) and thereafter outputs an adaptive command effort during subsequent samples to compensate for such modeling error and thereby tends to make the error signal e approach zero at the end of the seek or, equally speaking, to make the transducer 80 arrive at the target position at the end of the seek notwithstanding the modeling error. This approach was discussed in detail in co-pending patent application Ser. No. 09/539,349 pending that was filed on Mar. 31, 2000, entitled xe2x80x9cDISK DRIVE WITH ADAPTIVE CONTROL PATH THAT PRODUCES AN ADAPTIVE CONTROL EFFORT BASED ON A CHARACTERIZED DEVIATION BETWEEN ACTUAL AND MO[D]ELED PLANT RESPONSE,xe2x80x9d (the xe2x80x9cAdaptive Control Path Applicationxe2x80x9d), which application is commonly owned by the assignee of this application and hereby incorporated by reference in its entirety.
The Multi-rate Feed-Forward Application and the Adaptive Control Path Application represent significant advances. Neither, however, does anything to eliminate or reduce the modeling error between the modeled VCM response G0(z) and the actual VCM response G(z) in advance of actually starting a seek. That modeling error is embodied in the disk drive""s feed-forward control, and, more particular, in the relationship between the position reference signal and the feed-forward control signal that were cooperatively derived on the basis of the modeled VCM response G0(z).
There remains a need, therefore, for a method of tuning feed-forward control in a disk drive.
The invention may be regarded as a method of tuning a servo control system in a disk drive having a disk with servo control information recorded thereon, a transducer for reading the servo control information, a servo control system that produces a control signal by combining (1) a feed-forward command effort signal uffwd and (2) a feedback command effort signal ufb derived from a difference between (i) a target position that is controllably varied with a PES reference signal r and (ii) an indicated position that includes an indicated PES signal y derived from the servo control information read by the transducer; and a voice coil motor (VCM) characterized by an actual frequency response G(z) for positioning the transducer in response to the control signal; wherein the feed-forward command effort signal uffwd and the PES reference signal r are designed to be compatible with one another on the basis of a modeled frequency response G0(z) such that the PES reference signal r and indicated PES signal y vary in unison during a seek when the servo control system uses the feed-forward command effort signal uffwd to position the transducer from an initial track position to a final target position, and wherein the actual frequency response G(z) is different than the modeled frequency response G0(z) such that the feed-forward command effort signal uffwd and the PES reference signal r are not compatible with one another. In the preferred embodiment, the method comprises the steps of scaling the feed-forward command effort signal uffwd to substantially achieve a zero velocity condition at an end of the seek; scaling the feed-forward command effort signal uffwd to substantially achieve an on-target condition at the end of the seek; and modifying the PES reference signal r such that it substantially varies in unison with the indicated PES signal y during the seek.